On the Role of Spin-Orbit Coupling and State-Crossing Topography in the Nonradiative Decay of Ir(III) Complexes

A pillar of our current understanding of the photoluminescence of Ir(III) complexes is the assumption that the population of triplet metal-centered states determines an efficient nonradiative decay to the ground state minimum. Based on that assumption, the energy separation between the emitting state and the minimum-energy crossing point of the triplet metal-centered and ground states has been employed as a key variable for evaluating the ability of Ir(III) complexes to decay nonradiatively. We demonstrate that the strong spin-orbit coupling between the triplet metal-centered and ground state of Ir(III) complexes, together with the sloped topography of their crossing, leads to a significant energy separation between the two states, resulting in a reduced rate of nonradiative ground state recovery. Therefore, we propose that the role of metal-centered states is defined by the tendency of the excited state population to remain trapped in the metal-centered minima.